1. Field to which Invention Relates
The invention relates to an irradiation device for the ultraviolet range of the spectrum and more specifically for the UVA spectral range, that is to say for the wavelength range from 300 nm to 420 nm and consisting of a radiation source and a light guide.
2. The Prior Art
Such irradiation devices are used more specifically in dentistry in order to use UVA radiation for the polymerisation of dental resins, which are used for prevention, restoring and orthodontal purposes.
For such applications in the dental field, more especially for applications in situ the UVA irradiation device must comply with the following criteria.
1. The radiation intensity in the UV range must be sufficiently high in order to ensure, to take an example, that a wafer of composite material (for example Nuva Phil) with a diameter of 6 mm and a depth of 2 mm with a white or white-yellowish coloration cures satisfactorily within a period of radiation of 30 seconds at the most. The radiation intensity necessary for this purpose in the effective UVA range (wavelength lambda between 320 nm and 400 nm) should in this respect amount to at least 20 mW/cm.sup.2.
2. The radiation intensity should remain as constant as possible during the effective life of the UVA radiation source, since this will ensure that the same polymerisation time applies for the polymerisation of the same volume of composite material. This property is particularly important since if the UVA intensity decreases, complete and thorough curing and the durability of a dental filling cannot be guaranteed.
3. The UVA radiation source should immediately be ready for use after switching on, that is to say its full UVA power should be emitted and it should be capable of being switched on immediately directly after being switched off. This property makes it possible for the dentist to use the equipment in the most rational manner.
4. In no case should the UVA irradiation device emit radiation in a wavelength range below 320 nm as otherwise erythma and in some cases even malignant tissue changes may be inflicted on the patient and the dentist treating him. This criterion has in the United States of America even been made part of a statutory provision for such UVA irradiation devices by the Food and Drugs Administration. The constructional safety of the irradiation device should therefore be of such a high standard that even in the case of damage to the device no radiation escapes lying below 320 nm.
5. The UVA radiation source should have a low cost price as owing to its limited peiod of life it must be regarded as an expendable part of the irradiation device.
Irradiation devices which comprise an UV lamp and a rigid or flexible light guide have already been used in dentistry and are furthermore described in the following patent specifications by way of example: German (Offenlegungsschrift) No. 2,315,721, (Patentschrift) 2,206,424, Swiss Pat. No. 530,798 and U.S. Pat. No. 3,638,312.
Prior art irradiation devices comprise a gas discharge lamp as the UV light source as for example a mercury vapor discharge lamp. As UVA light guides transparent to UVA use is made of rigid light guides of quartz glass or flexible quartz fibre bundles and also liquid-plastics light guides.
These known irradiation devices leave much to be desired as regards their practical utility.
While known irradiation devices provide a sufficiently high UVA intensity for polymerisation, especially owing to the use of a gas discharge lamp as a light source they have the following disadvantageous properties. After operation for approximately 100 hours there is a substantial decrease in the UVA output, this applying particularly for the 365 nm Hg-line which is important for the polymerisation. After several hundred hours of operation the UVA output may have dwindled to only a third of the original amount. This leads to substantially longer polymerisation times. A further disadvantage of these known UVA irradiation devices resides in that the Hg lamps require a heating time of several minutes and after being switched off can only be struck again after a pause of a few minutes. Although measures have been proposed to accelerate the evaporation of mercury and accordingly to reduce the warm up time of a Hg high pressure lamp to approximately 20 seconds, such measures are technically involved and expensive. In the case of most devices with a Hg lamp the dentist must keep the device continuously in a ready state and the actual period of use only amounts to a very small fraction of the time which the device is actually switched on. A consequence of this is that after only a few hours of effective use time the UVA intensity substantially decreases.
Gas discharge lamps, more particularly Hg lamps, also have an intensive emission below 320 mm. This radiation must be cut out by means of external filters and the housing for the lamp must therefore be designed so as to be fully radiation-tight, something which makes it more difficult to remove dissipated heat. This latter circumstance applies more particularly for manually held irradiation devices with a rigid light guide. Damage to the radiation filter, which is generally arranged in front of the light entrance face of the light guides can lead to the emission of hard UV radiation without this being noticed. Finally it is to be mentioned that the gas discharge lamps as used in the case of prior art devices have a very high cost price.